1. Field of the Invention
The present invention relates to a decode circuit for decoding a multi-bit digital signal to produce an electric signal (voltage) corresponding to the multi-bit digital signal, and particularly to a decode circuit used in digital-to-analog conversion of converting the multi-bit digital signal to an analog signal, as well as to a display device supporting the decode circuit. More specifically, the invention relates to a configuration of a decode circuit in a digital-to-analog converter producing a pixel write voltage corresponding to input pixel data in an image display unit.
2. Description of the Background Art
For selecting one output candidate from a plurality of output candidates, a decode circuit is generally utilized. By utilizing a digital signal of n bits, one candidate can be selected from possible outputs of the n-th power of 2 in number, and a circuit layout area can be reduced, as compared with a configuration in which select signals are applied corresponding to the respective output candidates.
The construction of the decode circuit differs depending on the application of the decode circuit. For example, where one out of a plurality of signal lines is to be driven to a selected state as is performed, e.g., in an address decode circuit of a memory circuit, the decode circuit, such as a NAND type decode circuit, utilizing a logic gate is employed. According to the combination of bit values of n-bit digital signals, one of the plurality of signal lines is driven to the selected state depending on output signals of the logic gates.
When one electric signal is to be selected for output from a plurality of electric signals (currents or voltages), a ROM-type decode circuit using a switching matrix is employed. Switching elements in the switching matrix are selectively turned on to determine a route of the one electric signal in accordance with an input multi-bit signal. The connection between the switching elements and the input multi-bit signal is uniquely and fixedly established, and the relationship between the one/off states of the switching elements and the corresponding input signal bits is uniquely determined.
ROM-type decode circuit, as described above, is often utilized as a lookup table or the like, and is also used in a specific application as a digital-to-analog converter circuit for converting an input multi-bit signal (i.e., a digital signal formed of multiple bits) to an analog signal (voltage). Reference voltages corresponding to the respective levels which the input multi-bit signal can represent are prepared. Through a decoding operation, the reference voltage corresponding to the value of an applied multi-bit signal is selected. The values represented by the input multi-bit signal are discrete values, and the reference voltage levels are also discrete. Such digital-to-analog converter circuit is used, e.g., in a drive circuit for generating write voltages of pixels in a liquid crystal display unit. A reference voltage is selected corresponding to input pixel data, and the selected reference voltage is written onto a pixel electrode of the display pixel such as a liquid crystal element. Where the display element is the liquid crystal element, the brightness of the pixel is set according to the voltage between pixel electrodes, so that the liquid crystal element can represent an intermediate value between white and block, and gradation display can be implemented. Such liquid crystal elements are provided corresponding to red (R), green (G) and blue (B) so that the gradation display of color images can be achieved.
When the pixel data is n-bit data, the gradation display can be achieved at levels of n-th power of 2. Correspondingly, reference voltage levels of n-th power of 2 are required. For example, in the case of n=6, 6-th power of 2 is 64, and each of red (R), green (G) and blue (B) can be displayed at 64 levels, and multicolor display corresponding to 260 thousand colors can be implemented. In the case of (n=8), each of red (R), green (G) and blue (B) can be displayed at 256 (=eighth power of 2) levels, and multicolor display corresponding to 19.77 million colors can be implemented.
A digital-to-analog converter circuit for one color will now be discussed. In the case where the digital-to-analog converter circuit is implemented in a ROM-type decode circuit, and a switch matrix is used, for each reference voltage, switching transistors and receiving the respective input signal bits are connected in series. In this arrangement, the switching elements of n×(2^n) in number are required, where “^” represents a power, and the layout area of the decode circuit increases. Therefore, where a drive circuit is formed to be integrated with a display panel on the same chip, the chip real estate is greatly increased, which significantly impairs reduction in size of the display device.
Constructions aimed at reduction of layout areas of the digital-to-analog converter circuits are disclosed in Japanese Patent Laying-Open Nos. 2000-242209, 2000-066642 and 2003-029687 (References 1, 2 and 3, respectively).
According to the construction disclosed in Reference 1, gradation select units are provided corresponding to respective reference voltages. Each gradation select unit is comprised of a series connection body formed of a plurality of switching elements which are selectively turned on in accordance with a combination of bits of input pixel data, and these gradation select units are connected to a common output line (column line). The switching element is formed of a P- or N-channel MOS transistor (insulated gate field effect transistor). The MOS transistor is formed of a TFT (thin film transistor), and a well region for isolation of the P- and N-channel MOS transistors is not required to reduce a layout area of the circuit. Since the gradation select unit is simply formed of the series connection body of the switching elements, the elements are reduced in number as compared with a gradation select unit formed of a select switch, a latch circuit and a decode circuit, and the circuit layout area is reduced.
In the construction disclosed in the Reference 2, bits of input pixel data are divided into upper data bits and lower data bits. Reference voltage lines are provided corresponding in number to the gradation levels that can be represented by the upper bit group. Reference voltage select timing is set according to the value represented by the lower bit group, and the voltage level of each reference voltage line is adjusted such that the levels of the reference voltages rise by one quantum step. The upper bit group decode circuit is rendered active according to a select timing signal based on a decode result of the lower bit group, and the reference voltage determined by the select timing signal is selected and transmitted to an output line (a column line connected to the pixel element). A starting reference voltage is selected by the upper bit group, the reference voltage level is shifted according to the timing corresponding to the value of the lower bit group, the upper bit group decode circuit is activated according to such timing, and the upper bit decode circuit selects the corresponding reference voltage thus shifted. It is intended to reduce the number of the switching elements forming the gradation select unit. In the gradation select unit of the upper bit group decode circuit, the switching element is formed of a P- or N-channel MOS transistor.
In the construction disclosed in the Reference 3, input pixel data is divided into upper and lower bit groups, and the upper bit group selects the reference voltage line. The lower bit group is used for adjusting an active period of time of a select signal of an upper bit decode circuit. The voltage level of each reference voltage line starting from a start reference voltage is updated every predetermined time by one quantum step. The voltage level written onto the pixel data line (column line) is finally driven to the voltage level corresponding to the input pixel data. In the gradation select unit, the P- or N-channel MOS transistors are merely connected in series. Outputs of the respective gradation select units are commonly connected to an output line. Therefore, the construction in this Reference 3 includes a series connection body of the switch transistors responsive to the upper bits of the pixel data bits and the switching transistors responsive to the select signal provided by the lower data bit group, whereby a latch circuit and others are not required, and it is intended to reduce the number of the elements to reduce the layout area of the digital-to-analog converter circuit.
In the constructions disclosed in the References 1 to 3, MOS transistors of a single conduction type (P-channel MOS transistors or N-channel MOS transistors) are employed for the analog switches transmitting the reference voltages. For suppressing the influence of the threshold voltage of MOS transistors to accurately transmitting the reference voltages, therefore, it is necessary to provide a larger signal amplitude than in the case of utilizing an analog switch of a CMOS construction. This causes a problem of increase in current consumption of a circuit producing a control signal (select signal) corresponding to input pixel data. For obtaining a sufficient driving power of the MOS transistors, it is necessary to increase a channel width of the switching element, and this reduces the advantageous effect of reducing a layout area by the reduction in element number.
In the construction disclosed in the Reference 1, the transistors in the output stage of each gradation select unit are commonly connected to the output line. The gradation select units are arranged for the respective reference voltage lines, and the final stages (transistors nearest to the output line) are turned on according to the pixel data bit. Accordingly, a half of the final stage MOS transistors in the gradation select units are turned on, and on-capacitances of a large number of MOS transistors are connected to the output signal line so that its parasitic capacitance increases.
In the construction disclosed in the Reference 2, the reference voltage lines are prepared according to the number of gradation levels that can be represented by the upper bits, and one reference voltage line is selected according to the timing depending on the decoding result of the lower bits and the reference voltage level is successively raised. Therefore, when the change timing of the reference voltage deviates from the timing of the select timing signal resulting from the lower bit decoding, such a problem occurs that the reference voltage at a fully accurate level cannot be transmitted. Each reference current must be changed one quantum step at a time over the gradation levels which correspond in number to the lower bits, and this complicates the construction of the reference voltage generating unit. The gradation select unit includes a latch circuit for latching the decode result as well as the select switch for selecting the reference voltage line according to the output signal of the latch circuit to connects the selected line to the output line. This results in a problem that the elements in the gradation select unit are large in number, and the circuit layout area is large.
In the construction disclosed in the Reference 3, the gradation select unit likewise uses the P- or N-channel MOS transistors for the switching elements, and therefore, it is required to increase the amplitude of the signals for controlling on/off of the switching elements, so that the power consumption is large, similarly to Reference 1. Further, the select signal produced through decoding of the lower bits is commonly supplied to the transistors connected to the output line. Therefore, such a state is present that the switching elements of the gradation select units are commonly turned on. Thus, such a period of time is present that the parasitic capacitance of the output line is increased to cause a problem that the gradation level on the output line cannot be updated fast according to the input pixel data. Since the MOS transistors are used for the switching elements, the large layout area is required for increasing the driving power of the switching elements.
In the display devices employing the decode circuits disclosed in these References 1 to 3, the pixels are greatly increased in number for achieving higher definition, and the layout areas of the digital-to-analog converter circuits and particularly the pitches are made small. Therefore, even if a large number of MOS transistors are implemented, e.g., by TFTs (Thin Film Transistors), the switching transistors of the pixels are large in number. In order to lay out the digital-to-analog converter circuit along the direction of a smaller pitch, the size of a longitudinal direction must be increased, for example, through a layout of arranging the switching transistors, which are usually arranged on one line, on two lines. This extremely restricts the degree of freedom in layout, to cause a problem that the efficient circuit design is difficult to achieve.
In the case where the P- or N-channel MOS transistor is used as the switching element for the gradation selection and the amplitude of the control signal is increased, dielectric-breakdown voltage characteristics deteriorate, and the element durability is adversely affected. As a countermeasure, an analog switch of a CMOS type may be simply used for the switching element, a greater load capacitance (on-capacitance) comes to be parasitic on the output line, and it becomes impossible to perform a fast decode operation. Further, the total number of switching elements is great, and it becomes difficult to perform an efficient layout within a small area, and yield lowers in product manufacturing.
The construction of the decode circuit described above can be applied to the circuits other than the digital-to-analog converter circuit for producing the analog voltage according to the input digital data, and it can be applied, e.g., to a switch matrix circuit establishing a transmission path of a certain signal by a decode circuit, and in such a case, similar problems would occur.